1. Field of the Invention
The present invention relates to a method for producing, with a high productivity, a glass preform for use in the fabrication of an optical fiber having a low transmission loss.
2. Description of the Related Art
In the optical fiber field, a deposited mass of glass soot which is synthesized by vapor phase synthesis such as a VAD method or an OVD method is usually consolidated in an electric furnace under atmospheric pressure. In particular, to reduce the transmission loss of the finally fabricated optical fiber, water contained in the soot preform is removed before consolidation. For example, Japanese Patent Kokai Publication No. 14936/1975 discloses a method for dehydration and consolidation of a soot preform in a gas mixture of helium and about 10% by volume of chlorine. In the conventional methods, the soot preform, namely a porous glass preform is generally consolidated under atmospheric pressure.
A method for consolidating the soot body under in vacuo is disclosed in Japanese Patent Kokai Publication No. 201025/1988, which method uses a heating apparatus comprising a vacuum container having a heater and a muffle tube within the heater and heats the soot preform in the muffle tube.
The conventional methods which consolidate the soot preform in an atmosphere of helium or a mixture of helium and a halogen gas under atmospheric pressure have good water-removing ability if contamination of the atmosphere with water or impurities is prevented, and is an excellent method for the production of high purity quartz glass.
Recently, to increase the productivity, a size of the preform has been increased. When a diameter of the preform is increased, bubbles tend to remain in a center part of the preform. To prevent this problem, the consolidation of the large size soot preform should be effected slowly. Therefore, the increase of the productivity of the soot preform does not lead to the increase of the productivity of the transparent glass preform, so that increase of the diameter of the preform is limited.
In the method for consolidating the soot preform in vacuo, since the vacuum container is required to have good sealability and pressure-resistance, it should be made of a metal. But, since the muffle tube is contained in the container, the sealing cannot be complete, so that an outer metal wall is corroded and a metal halide is formed and contaminates the soot preform. Therefore, clean dehydration cannot be achieved.